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Diffusion tensor imaging of white matter tracts in patients with cerebral neoplasm PowerPoint Presentation

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Published on : Jul 28, 2021 | Views : 1691 | Downloads : 4

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Slide 1-

Diffusion tensor imaging of white matter tracts in patients with cerebral neoplasm. Dr/ ABD ALLAH NAZEER. MD.
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The Physics of DTI By applying the appropriate magnetic field gradients, MR imaging may be sensitized to the random, thermally driven motion (diffusion) of water molecules in the direction of the field gradient. Diffusion is anisotropic (directionally dependent) in WM fiber tracts, as axonal membranes and myelin sheaths present barriers to the motion of water molecules in directions not parallel to their own orientation. The direction of maximum diffusivity has been shown to coincide with the WM fiber tract orientation . This information is contained in the diffusion tensor, a mathematic model of diffusion in three-dimensional space. In general, a tensor is a rather abstract mathematic entity having specific properties that enable complex physical phenomena to be quantified. In the present context, the tensor is simply a matrix of numbers derived from diffusion measurements in several different directions, from which one can estimate the diffusivity in any arbitrary direction or determine the direction of maximum diffusivity. The tensor matrix may be easily visualized as an ellipsoid whose diameter in any direction estimates the diffusivity in that direction and whose major principle axis is oriented in the direction of maximum diffusivity. With use of DTI, both the degree of anisotropy and the local fiber direction can be mapped voxel by voxel, providing a new and unique opportunity for studying WM architecture in vivo.
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The tensor model of diffusion consists of a 3 × 3 matrix derived from diffusivity measurements in at least six noncollinear directions. The tensor matrix is diagonally symmetric (Dij = Dji) with six degrees of freedom (ie, only six of the tensor matrix’s nine entries are independent and so the matrix is fully determined by these six parameters), such that a minimum of six diffusion-encoded measurements are required to accurately describe the tensor. Using more than six encoding directions will improve the accuracy of the tensor measurement for any arbitrary orientation. The tensor matrix is subjected to a linear algebraic procedure known as diagonalization, the result of which is a set of three eigenvectors representing the major, medium, and minor principle axes of the ellipsoid fitted to the data and the corresponding three eigenvalues (λ1, λ2, λ3), which represent the apparent diffusivities along these axes. (The word eigen is Germanic in origin, meaning “peculiar” or “special.” The term eigenvalue was used by British algebraists in the late 19th century to refer to a “characteristic value” of a matrix; specifically, a number k is called an eigenvalue of the matrix A if there exists a nonzero vector v such that Av = kv. In this case, the vector v is called an eigenvector of A corresponding to k. This procedure may be thought of as a rotation of the x, y, and z coordinate system in which the data were acquired (dictated by scanner geometry) to a new coordinate system whose axes are dictated by the directional diffusivity information.
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Diffusion tensor imaging (DTI) is a non-invasive method for visualizing white matter tracts in the brain. Tractography can now be easily performed using clinical magnets and the degree of anisotropy and the local architecture of white matter changes can be assessed. This technique provides unique anatomic information by reconstructing and visualizing chosen fiber tracts in 3-dimensional (3D) anatomy of the brain. The aim of surgical treatment of brain tumors is the maximum range of tumor resection while minimizing postoperative neurological deficits resulting from iatrogenic brain damage. This requires preoperative or intraoperative mapping of the tumor and its relation to important functional areas, including the cerebral cortex and white matter tracts. Brain mapping can be performed either by using a functional MR imaging or intraoperative cortex electrostimulation. These methods are inadequate to assess the relationship of tumor and white matter tracts. DTI is uniquely suited for this role. There are some potential interactions of the white matter fibers and brain tumors, fibers could be edematous, deviated, infiltrated or destroyed by tumor. The extent to which these patterns can be distinguished on the basis of the DTI is still not fixed and is the subject to further evaluation. The aim of the work is preoperative assessment of the tumor and tumor like lesions and white matter tracts relationship in the context of the histological type of the tumor using DTI in the pediatric patients with brain tumors.
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WM Fiber Classification. WM fiber tracts traditionally have been classified as follows: Association fibers interconnect cortical areas in each hemisphere. Fibers of this type typically identified on DTI color maps include cingulum, superior and inferior occipitofrontal fasciculi, uncinate fasciculus, superior longitudinal (arcuate) fasciculus, and inferior longitudinal (occipitotemporal) fasciculus. Projection fibers interconnect cortical areas with deep nuclei, brain stem, cerebellum, and spinal cord. There are both efferent (corticofugal) and afferent (corticopetal) projection fibers. Fibers of this type typically identified on DTI color maps include the corticospinal, corticobulbar, and corticopontine tracts, as well as the geniculocalcarine tracts (optic radiations). Commissural fibers interconnect similar cortical areas between opposite hemispheres. Fibers of this type typically identified on DTI color maps include corpus callosum and anterior commissure. Other tracts that are occasionally, but not consistently, identified on directional DTI color maps include optic tract, fornix, tapetum, and many fibers of the brain stem and cerebellum. Space limitations preclude a comprehensive review of all tracts potentially visualized with DTI. Rather, we focus on the major tracts that are consistently identified in our practice.
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The DTI protocol included 60 slices, 2.0 thickness, FOV 224 mm, matrix 128x128, TR/TE 1000/74-79, 20 diffusion directions, b value=0 and 800 s/mm2. Further DTI data processing was performed using dedicated manufacturer software Fiber Trak. 3D fiber tractography and color-coded fractional anisotropy maps were performed and evaluated how the fiber tract were changed in brain lesions. On the basis of the literature and own observations the authors distinguished 5 types of the tumor relation to the surrounding white matter fibers: fibers in the anatomic location, correct, fibers displaced, spaced apart and the value of the anisotropy in the tumor is normal or slightly reduced, on a colorful map FA color fiber unchanged, only the brightness changed III-fibers displaced, FA reduced, on a colorful FA map fibers color partially changed fibers infiltrated, numerous crossing fibers, spurious fibers, increased or decreased FA values. fibers destroyed, anisotropy of about 0.
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A, FA map without directional information. B, Combined FA and directional map. Color hue indicates direction as follows: red, left-right; green, anteroposterior; blue, superior-inferior. This convention applies to all the directional maps in this review. Brightness is proportional to FA. Fiber Tract Anatomy.
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Tractograms of the superior (C) and inferior (D) occipitofrontal fasciculi.
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The paired cingula (arrowheads) are easily identified in green on this section obtained just cephalad to the corpus callosum. B, Inferior occipitofrontal fasciculus (white arrows) and inferior longitudinal fasciculus (yellow arrow), axial directional map.
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Projection Fibers: Corticospinal, Corticopontine, and Corticobulbar Tracts.
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C, Directional map, three adjacent parasagittal sections, with corona radiata identifiable in blue (arrows). Corona radiata fibers interdigitate with laterally directed callosal fibers, resulting in assorted colors in the vicinity of their crossing. D, Tractogram in which different portions of the corona radiata have been parsed by initiating the tractographic algorithm from different starting locations.
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Geniculocalcarine tract (optic radiation), axial view. A–D, Illustration (A), gross dissection (B), directional map (C), and tractogram (D). As this tract connects the lateral geniculate nucleus to occipital (primary visual) cortex, the fibers sweep around the posterior horn of the lateral ventricle and terminate in the calcarine cortex (more cephalad fibers of the optic radiation take a more direct path to the visual cortex). The optic radiation (arrows) mingles with the inferior occipitofrontal fasciculus, inferior longitudinal fasciculus, and the inferior aspect of superior longitudinal fasciculus to form much of the sagittal stratum in the occipital lobe.
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A and B, Sagittal directional map of the corpus callosum (arrowheads) (A) and tractogram (B). (A), gross dissection (B), directional map (C), and tractogram (D). The largest WM fiber bundle, the corpus callosum connects corresponding areas of cortex between the hemispheres. Close to the midline, its fibers are primarily left-right oriented, resulting in its red appearance on this DTI map. However, callosal fibers fan out more laterally and intermingle with projection and association tracts, resulting in more complex color patterns.
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DTI Patterns in WM Tracts Altered by Tumor: The goal of surgical treatment for cerebral neoplasms is to maximize the extent of tumor resection while minimizing postoperative neurologic deficits resulting from damage to intact, functioning brain. This requires preoperative or intraoperative mapping of the tumor and its relationship to functional structures, including cerebral cortex and WM tracts. Cortical mapping can be accomplished with either functional MR imaging or intraoperative electrocortical stimulation. These methods are inadequate, however, for depicting the relationship of tumor to WM tracts. DTI is uniquely suited for this role. The altered states of WM resulting from cerebral neoplasm might be expected to influence the measurement of diffusion tensor anisotropy and orientation in various ways, resulting in several possible patterns on directional DTI color maps. Intact WM tracts displaced by tumor might retain their anisotropy and remain identifiable in their new location or orientation on directional DTI color maps. Edematous or tumor-infiltrated tracts might lose some anisotropy but retain enough directional organization to remain identifiable on directional DTI maps. Finally, WM tracts might be destroyed or disrupted to the point where directional organization (and, consequently, diffusion anisotropy) is lost completely.
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MRI showing a tumor FT showing location of tumor in relation to surrounding tracts
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DTI pattern 1: normal anisotropy, abnormal location or orientation. A–E, T2-weighted MR image (A), contrast-enhanced T1-weighted image (B), directional maps in axial (C) and coronal (D) planes, and coronal tractogram of bilateral corticospinal tracts (E). WM tracts are deviated anteriorly, inferiorly, and posterolaterally by this ganglioglioma but retain their normal anisotropy. Therefore, they remain readily identified on DTI (C and D) and readily traced with tractography (E). The AC (red, arrowhead), IOFF (green, open arrow), and CST (blue, solid arrows) are deviated. Note the blue hue of the CST change to red as it deviates toward the axial plane by the tumor (arrow on coronal view [D]).
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DTI pattern 2: abnormal (low) anisotropy, normal location and orientation. Anisotropy, normal location and orientation. A–D, T2-weighted MR image (A), contrast-enhanced T1-weighted MR image (B), FA map (C), and directional map (D). The homogeneous region of hyperintensity on the T2-weighted image represents vasogenic edema surrounding a small metastasis (on another section, not shown). Despite diminished anisotropy in this region (darker region outlined on FA map) and diminished color brightness on directional map, the involved fiber tracts retain their normal color hues on the directional map (superior longitudinal fasciculus, green, arrow; corona radiata, blue, arrowhead). This preservation of normal color hues despite a substantial decrease in anisotropy is consistent with the abnormality of vasogenic edema, which enlarges the extracellular space (allowing less restricted diffusion perpendicular to axonal fibers, thus reducing the anisotropy) without disrupting cellular membranes, leaving their directional organization intact. It is not yet known to what extent this pattern is specific for edema, however.
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DTI pattern 3: abnormal (low) anisotropy, abnormal orientation. Abnormal (low) anisotropy, abnormal orientation. A–D, T2-weighted MR image (A), contrast- enhanced T1-weighted image (B), FA map (C), and directional map (D). This infiltrating astrocytoma is characterized by both diminished anisotropy and abnormal color (arrowhead) on the directional map, suggesting disruption of WM fiber tract organization more severe and complex. Note that the color change cannot easily be attributed to bulk mass effect as in purely deviated tracts.
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DTI pattern 4: near-zero anisotropy, tract unidentifiable. A–D, T2-weighted MR image (A), contrast- enhanced T1-weighted image (B), FA map (C), and directional map (D). This high-grade astrocytoma has destroyed the body of the corpus callosum, rendering the diffusion essentially isotropic and precluding identification on the directional map (arrow).
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T2-weighted (T2W), ADC, FA, and FA-weighted, directionally encoded DTI color maps (FA 1) in a patient with a region of bland (tumor-free) vasogenic edema involving the left corona radiata and superior longitudinal fasciculus.16 This region shows reduced FA, but normal color hues (arrow).
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T2-weighted (T2W), ADC, FA, and FA-weighted, directionally encoded DTI color maps (FA . ε1) in a patient with an infiltrating glioma involving the left corona radiata and subcortical U-fibers.16 This region shows both reduced FA and abnormal color hues (arrow), which are not explained by any apparent tract deviation.
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A, T2 weighted image) Brain-tumor-related edema (hyperintense area) characterized by increased diffusivity (b, trace ADC map) and reduced anisotropy (c, FA map). White matter of fiber directionality is not apparent within the edematous region (c, FA map, and d, color-coded FA map). After treatment, the edema was resolved (e, T2 weighted image) and the trace ADC, FA, and color-coded FA shows normal appearance of the diffusion indices the acute phase of the edema, one could suspect that neuronal fibers might be damaged within the edematous region. However, the follow-up examination shows that the reduced anisotropy in the acute phase only represents in-direct effect of the edema and not the condition of the white matter fibers.
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Image of fusion of magnetic resonance imaging and tractography in a patient harboring a right frontal high grade glioma.
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Thirty-six-year old male patient with an anaplastic astrocytoma at the left insula. The tumor mass shows poorly demarcated high signal intensity on T2-weighted MR images (A) and low signal intensity on T1-weighted MR images (B). During preoperative surgical planning (C), the arcuate fasciculus (red) was suspicious of partly passing through the tumor mass (purple) and the corticospinal tract (yellow) was surrounding the mass. When the superior part of the tumor was resected in an awake state (D), aphasia developed, and we stopped further resection of the tumor. Postoperative MRI with diffusion tensor image shows small remnants of tumor and a well preserved arcuate fasciculus (E).
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Eighteen-year-old female patient with a pilocytic astrocytoma. The preoperative T1-weighted MR image shows a well enhanced cystic mass at the right occipital area (A). Navigation snap shot image (B) during operation reveals that the optic radiation is passing the medial margin of the tumor. Postoperative MRI with diffusion tensor image shows completely resected tumor and a well preserved optic radiation (C).
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Right temporal ganglioglioma A: motor fMRI; B) 3D-SPGR; C: motor fMRI (red) fused with DT-tractography (yellow)
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Case of white matter tracts displacement. A. Preoperative computer tomography before (left and after (right) contrast administration showing the parietal tumor (glioblastoma). B and C. 3D reconstruction DTI showing displacement of the tracts surrounding the tumor.
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Case of white matter tracts disruption ,T1- weighted(left) and enhanced T1-weighted (right) preoperative R image showing the temporal tumor (glioblastoma) C. 3D DTI reconstruction showing disruption of the tracts in the cerebral volume occupied by the tumor.
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A 71-year-old male with a glioblastoma in the left thalamus. Axial contrast- enhanced T1-weighted image (a) shows solid enhancement. MD map (b) shows restricted diffusion of the enhancing part (0.75 × 10−3/mm2/s). FA (c), CL (d), and CP (e) from the enhancing part (0.18, 0.15, and 0.15, respectively)
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A 53-year-old male with metastatic lung adenocarcinoma in the left frontal lobe. Axial contrast-enhanced T1-weighted (a) shows a solid enhancing lesion. MD map (b) shows restricted diffusion of the enhancing part (0.95 × 10−3/mm2/s). Lower FA (c), CL (d), and CP (e) are noticed from the enhancing part (0.10, 0.08, and 0.09, respectively).
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58-year-old female with primary cerebral diffuse large B cell lymphoma in the right peritrigonal area. Axial contrast-enhanced T1-weighted (a) shows a solid enhancing lesion with extensive edema. MD map (b) shows restricted diffusion of the enhancing part (0.80 × 10−3/ mm2/s). Lower FA (c), CL (d), and CP (e) are noticed from the enhancing part (0.08, 0.08, and 0.06, respectively).
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Tumor/white matter relationship pattern I. T2-weighted MR image shows high signal intensity lesion in the left putamen , FA map reveals normal anisotrophy results in right and left putamen. It was inflammatory lesion.
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Tumor/white matter relationship pattern II. FA map reveals large ganglioglioma in the fourth ventricle with displaced fibers, FA values decreased
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FA map shows low FA value in the left temporal lobe tumor (pilocytic astrocytoma) and directional map reveals spurious fibers and infiltrated fibers
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Tumor/white matter relationship pattern III. FA directional map reveals pilocytic astrocytoma in the right temporal lobe with fibers color partially changed, FA values decreased
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Tumor/white matter relationship pattern IV. FA directional map reveals medulloblastoma in the right cerebellar hemisphere with infiltrated fibers, FA values decreased.
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Tumor/white matter relationship pattern V. FA directional map reveals large scar with destroyed fibers, rendering the diffusion essentially isotropic. Tractogram reveals large scar with destroyed fibers.
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Ipsilateral corona radiata before(a) and after (b) tumor. Fiber trajectories were color-coded accordingly to their anterior-posterior position. Their relative position is shown in axial FA maps. A FA threshold of 0.08 was used for terminating fiber trajectories. Continuous trajectories were obtained even through regions of low anisotropy (indicated by arrows).
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DTI: destruction and deviation of white matter tracts by anaplastic astrocytoma.
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Differential response of tumour to chemotherapy with tractography.
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FA maps and tractography are helpful in the planning of surgical treatment of brain tumors. Our data may be interesting for future studies using DTI to prospective analyse of the relation between brain tumors and white matter tracts. The mass effect of the tumor can cause large changes in the main directions of diffusion around the tumor resulting in false tractography 3D reconstruction. Therefore, preoperative assessment of the relationship between tumor and white matter tract should not be assessed only by using tractography but also with maps FA - normal and color coded. Conclusion
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Thank You.